How to make a complaint against a walmart employee?

Yesterday at Walmart my niece had wandered away from my aunt while they were shopping and when my aunt had went up to the "costumer service" desk to ask if they could please call for her daughter they completely said "yeah, hold up." ignored her request, and she stood there for 10 good minutes without them doing anything but laughing and chitter-chatting with each other. How could you file a complaint? I tried to go online but couldn't find anything but how to complain about the website setup not an employee at a certain store.. is there a number or something I should call?